GOP Sen. Lamar Alexander won't seek reelection

Sen. Lamar AlexanderAndrew (Lamar) Lamar AlexanderWhite House proposes limits on student loan borrowing as part of higher education reforms The 25 Republicans who defied Trump on emergency declaration Trump issues first veto, warning of 'reckless' resolution MORE (R-Tenn.) on Monday announced that he will not run for reelection in 2020.

"I will not be a candidate for re-election to the United States Senate in 2020. The people of Tennessee have been very generous, electing me to serve more combined years as Governor and Senator than anyone else from our state," Alexander said.

Alexander, who has been in the Senate since 2003, said he will serve out the remainder of his term, which runs through the end of 2020. 

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"I have gotten up every day thinking that I could help make our state and country a little better, and gone to bed most nights thinking that I have. I will continue to serve with that same spirit during the remaining two years of my term," he added.

Alexander, 78, is the first senator up in 2020 to announce that they won't seek reelection. Republicans face a challenging map during the next cycle, where they'll be defending approximately 22 seats, including in states like Colorado and Maine, which Democratic presidential nominee Hillary ClintonHillary Diane Rodham ClintonDem lawmaker says Electoral College was 'conceived' as way to perpetuate slavery Dem strategist says Donna Brazile is joining Fox News 'for the money' CNN to host town hall with Cory Booker in South Carolina MORE won in 2016.

Alexander is also the latest in a string of GOP chairmen from the establishment wing of the party to announce that they will retire during the Trump era: Senate Finance Committee Chairman Orrin HatchOrrin Grant HatchNY's political prosecution of Manafort should scare us all Congress must break its addiction to unjust tax extenders The FDA crackdown on dietary supplements is inadequate MORE (R-Utah) and Senate Foreign Relations Committee Chairman Bob CorkerRobert (Bob) Phillips CorkerTrump keeps tight grip on GOP Brexit and exit: A transatlantic comparison Sasse’s jabs at Trump spark talk of primary challenger MORE (R-Tenn.) are retiring at the end of the current Congress.

Corker, on Monday, called working with Alexander "one of the highlights" of his work in the Senate.

"As one of the finest statesmen our state has ever seen, Lamar will leave behind a remarkable legacy. I know he will press through the next two years with great vigor, and I look forward to all he will accomplish on behalf of Tennesseans as he completes his service in Washington," Corker said in a statement.

Alexander previously served as chairman of the Senate Republican Conference and is considered to be close to both Majority Leader Mitch McConnellAddison (Mitch) Mitchell McConnellGOP rep to introduce constitutional amendment to limit Supreme Court seats to 9 The Hill's Morning Report - Dems contemplate big election and court reforms Court-packing becomes new litmus test on left MORE (R-Ky.) and Minority Leader Charles SchumerCharles (Chuck) Ellis SchumerGOP rep to introduce constitutional amendment to limit Supreme Court seats to 9 Why we need to build gateway now Campaign to draft Democratic challenger to McConnell starts raising funds MORE (D-N.Y.).

He's also been at the center of some of the biggest fights in the Trump administration because of his position as chairman of the Health, Education, Labor and Pensions (HELP) Committee. 

He was part of a working group of GOP senators, assembled by McConnell, that crafted the Senate GOP's failed ObamaCare replacement bill last year. 

But Democrats have credited him with being willing to work with them on key issues despite growing partisanship in the Senate. 

He crafted a deal with Sen. Patty MurrayPatricia (Patty) Lynn MurrayWhite House proposes limits on student loan borrowing as part of higher education reforms Jury orders Johnson & Johnson to pay M to woman who claimed baby powder gave her cancer Overnight Health Care - Presented by Kidney Care Partners - FDA chief Scott Gottlieb resigns | House Dems to take up drug pricing bills next week | Planned Parenthood, doctors group sue over Trump abortion rule MORE (D-Wash.), the top Democrat on the health committee, to funds key payments to health insurers for two years.

He was also one of less than 10 GOP senators who voted for a bipartisan immigration proposal earlier this year that would have provided approximately 1.8 million immigrants brought to the United States illegally as children an eventual pathway to citizenship in exchange for $25 billion in border security. 

Sen. Ed MarkeyEdward (Ed) John MarkeySenate Dems petition Saudi king to release dissidents, US citizen The Hill's 12:30 Report: Manafort sentenced to total of 7.5 years in prison Hillicon Valley: Google takes heat at privacy hearing | 2020 Dems to debate 'monopoly power' | GOP rips net neutrality bill | Warren throws down gauntlet over big tech | New scrutiny for Trump over AT&T merger MORE (D-Mass.) on Monday also credit Alexander as being a "true partner" in addressing the country's opioid epidemic. 

Alexander's decision to retire will leave a hole in Tennessee politics, where he has been a mainstay for decades. He was first elected to the Senate in 2002, previously served as governor of Tennessee and made two unsuccessful bids for the White House in 1996 and 2000.

But he was considered potentially vulnerable to a primary challenge in 2020 after winning narrowly fending off tea party challenger Rep. Joe Carr during the 2014 GOP primary. 

It's unclear who will vie to succeed Alexander. Tennessee Gov. Bill Haslam (R) flirted with a Senate bid in 2018. And state Sen. Mark GreenMark GreenGOP rep to introduce constitutional amendment to limit Supreme Court seats to 9 The Memo: 5 takeaways from Cohen’s explosive day of testimony GOP lawmaker rips Cohen: ‘The truth is not in this guy’ MORE (R) hinted last year that he could make a Senate run in 2020.  

Green told USA Today in August that he would be "back in the election cycle" in 2020.

"I think that's going to be an opportunity for me potentially later," he added about a potential Senate run. 

President TrumpDonald John TrumpDem lawmaker says Electoral College was 'conceived' as way to perpetuate slavery Stanley Cup champion Washington Capitals to visit White House on Monday Transportation Dept requests formal audit of Boeing 737 Max certification MORE won Tennessee by more than 26 points in 2016. In last month's election to replace Corker, Rep. Marsha BlackburnMarsha BlackburnTaylor Swift says she wants to get more involved in politics Bipartisan lawmakers introduce resolution supporting vaccines Hillicon Valley: Cohen stuns Washington with testimony | Claims Trump knew Stone spoke to WikiLeaks | Stone, WikiLeaks deny | TikTok gets record fine | Senators take on tech over privacy MORE (R-Tenn.) defeated former Gov. Phil Bredesen (D) by nearly 11 points.

Updated at 1:12 p.m.